Detecting vehicle license plates in a camera field or in an image sequence (video stream) of a video camera is a common task in traffic monitoring applications and also a preliminary step for automatic license plate reading methods using optical character recognition (OCR). To obtain reliable recorded images that can be read well by OCR, the precise image in the image sequence to be detected is the image from which, the vehicle license plate can be read completely and well. Moreover, for OCR applications, it is advantageous if the vehicle license plate is located in the most possible stable position in the image. For this purpose, conventional license plate detection systems use “virtual trigger lines” that are defined in the optical camera image and when the license plate of a passing vehicle is detected at this trigger line, the next image(s) of the image sequence are used as a detection result for further OCR processing.
It is known to carry out the detection at the virtual trigger lines by uncoupling a light line from the optical camera image via glass fiber guides onto a separate detector line, or by real time image processing methods performed directly on the video stream. The first variant is mechanically/optically complicated, and the second variant is complicated in terms of computation of resources. Moreover, due to the limited frame rate of the usual video streams, for example, 25-50 frames per second (fps), the second variant leads to “position jitter” of the vehicle license plate in the image view selected as the detection result. Since the vehicles move over a road section and thus over the optical camera image at different speeds, they come to be located in the images of the image sequence in different positions relative to the virtual trigger line depending on their speed, which makes evidence and/or OCR evaluations of the resulting image more difficult.